#1EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Southern Methodist University Fall 2003 EETS 8316/NTU CC745-N Wireless Networks Lecture 8: Mobile Data,

#1EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU

Southern Methodist University Fall 2003

EETS 8316/NTU CC745-NWireless Networks

Lecture 8: Mobile Data, Part III

Instructor: Jila Serajemail: [email protected]

http://www.engr.smu.edu/~jseraj/tel: 214-505-6303


Session Outline

Review of last week

Wireless LAN


Announcements

Answer to homework #1 is on the web

Homework #2 is on the web.

—Deadline for in-campus students October 24

—Deadline for distant students November 7


Review, GPRS - Network Architecture

GPRS makes use of existing GSM base stations

Serving GPRS support node = packet switch with mobility management capabilities

Gateway GSN = packet switch interworks with other networks

Internet or other networks

GGSNMSC/VLR

SGSN SGSN

HLR

BSC/PCU


Review, GPRS , Cont...

GSM Release’97 introduced general packet radio service (GPRS) for bursty data

Make use of existing GSM network equipment and functions

In Contrast to CDPD, it is integrated into GSM, i.e. dedicated Control channel and data channel.

Requires two new network element, GGSN and SGSN



GGSN = Gateway GPRS Support Node

— External interfaces

— Routing

GPRS register maintains GPRS subscriber data and routing information. Normally it is integrated in GSM HLR

PCU (Packet Control Until) is collocated with BSC.



Three class of mobile terminals

—Class A: Operates GPRS and Circuit switched service simultaneously

—Class B: Monitors the Control channels of GPRS and GSM simultaneously but can operate one set of services at a time

—Class C: Only CS or GPRS capable.



For mobility management a new concept is defined, Routing Area

RAI = MCC +MNC + LAC + RAC


Review, GPRS Interfaces


Review, GPRS – Data Connection

GPRS data connection starts with Attach and ends with Detach.

Attach is the phase when the mobile informs the network of its intention to create a data connection

At conclusion of Attach, SGSN is ready to set up data services on behalf of the mobile user.


Review, GPRS – Data Connection, Cont…

Detach is the phase when mobile terminates the connection.

GPRS requires subscription


Review, GPRS Attach Scenario

BSS HLRSGSNBTS

IMSI, P_TMSI+OLD RAI…

Update Location

Insert Subs. Data

Insert Data Ack

Update LocationGPRS Attach Accepted


Review, GPRS – Mobile Attach Scenario

Mobile sends Attach message. This message contains P-TMSI or TMSI. It also contains NSAPI (Network Service Point Identifier)

SGSN contacts HLR to verify if the user is permitted to use the service

After authentication, SGSN send back Attach Accepted together with a TLLI (Temporary Logical Link Identity)


Review, GPRS – Mobile Attach Scenario

A database in SGSN is now populated with mobile identity and TLLI. TLLI is used by logical link controller in the SGSN.


Review, GPRS – Setting Up Packet Data

After attach the mobile is known by SGSN and have an identity there, but it is not known to the external network.

First it needs to create an identity for itself by performing a procedure called PDP Context Activation. PDP is Packet Data Protocol, which could be IP or x.25 protocol.


Review, PDP Context ActivationBSS GGSNSGSNBTS

Activate PDP ContextCreate PDP

Context RequestNASPI, PDP typePDP, QoS,APN

Create PDPContext ResponsePDP Address, QoS

Activate PDP Context AcceptedPDP Type, PDP Address, QoS


Review, PDP Context Activation, Cont..

Mobile requests PDP Context Activation

Based on the information provided, SGSN determines which GGSN to connect to. The GGSN should be capable to support the PDP requested by mobile

GGSN updates its data base and assign a TID to the mobile and SGSN

SGSN updates its data base with the GGSN address and TID. It then send PDP Context Activation Accepted message to mobile


Review, Actually Sending Data

After PDP Context Activation the mobile is known to the external packet network (PDN)

When SGSN receives data from mobile, it looks up its database and relate the TLLI to NSAPI.

SGSN and SNDPC pad the IP packet and replace the destination address with GGSN IP address and sets GTP header to TID


Review, Actually Sending Data, Cont…

Packets are then sent to GGSN with SGSN as sender

At GGSN, the additional information is removed to get the original packet . The packet can now be routed to its intended destination.


Wireless LANs

Wireless LANs are usually logical bus topology (broadcast medium)

Why wireless LANs?

—Saves trouble of rewiring a building

—Portable computing devices (laptops, PDAs) are more common


MAC Protocols

MAC protocol is a sublayer in data link layer

For LANs, data link layer = logical link control (LLC) sublayer + MAC sublayer

data link

physical

LLC

MAC

network

- defines how stationsaccess the sharedmedium


MAC Protocols, Cont..

—LLC sublayer builds on MAC sublayer to provide medium-independent communication service to higher layers (makes MAC sublayer transparent)

—LLC can provide appearance of connectionless or connection-oriented service


MAC Protocols, Cont..

•Connectionless service treats each message independently. No connection setup and no sequential order

•Connection-oriented service requires connection setup and preserves sequential order of messages


MAC protocols, Token Passing

Token ring and token bus—Every station connected to the bus is given

a token—The token is passed according to order—When a station has something to send, it

keeps the token until it is done, before sending it to the next station.

It is fair and has no contention

The system encounters delays for sending the token.


MAC Protocols: Token Passing, Cont..

Token passing is another technique to eliminate contention (collisions)

Token is short packet representing permission to transmit—Token is passed from station to station

according to an arranged order defining a logical token ring topology

—A station with the token can transmit for a limited time

—After transmission, token is sent to next station in ring


MAC Protocols: Polling

Objective to eliminate random contention (collisions) which reduces throughput of system

Polling is centralized control

—One station will periodically poll other stations to see if they have data to transmit

—A polled station may transmit data, otherwise controller will poll next station in a list



Polling involves exchange of control messages between stations and controller

—Efficient only if

• roundtrip propagation delay is small

•overhead due to control messages is small

•user population is not large and bursty

—As population increases with more bursty users, performance of polling degrades



Polling is used in wired network environments but not popular in wireless networks


Token passing, Cont..

Commonly used in wired LANs (IEEE 802.4 token bus and 802.5 token ring), token passing has not found much adoption in wireless networks

Overhead is increased to improve throughput under heavy load

—Issue is efficiency


MAC protocols: Aloha

Aloha

—Stations starts sending when they have something to send

—Pure Aloha, no contention resolution, relies on timed-out acks, max throughput 18%

—Slotted Aloha, no contention resolution, relies on timed-out acks, only can start sending in the beginning of a slot, max through put 36%


MAC Protocol: Pure ALOHA, Cont..

Throughput

—Assume infinite population of stations generating frames at random times

—Each frame is transmitted in fixed time T

—Assume average number of transmission attempts is S in any interval T



Throughput

—Number of new transmission attempts in any interval t has Poisson probability distribution:

Pr(k transmissions in interval t ) = (St)ke- St /k!

—Let G = “offered load” = new transmissions and retransmissions



—In equilibrium, throughput (rate of successfully transmitted frames) = rate of new transmissions, S

S = GP0

where P0 = probability of successful transmission (no collision)

—P0 depends on “vulnerable interval” for frame, 2T



frame A

frame B

frame C

time-T 0 T

- transmission attempt at time 0

- collision if starts in interval (-T,0)- collision if starts in interval (0,T)



P0 = Pr(no other frame in 2T interval)

—Assume total number of frames in any interval t is also Poisson distributed, with average G:

Pr(k transmissions in t) = (Gt)ke-Gt/k!

then P0 = e-2G

—By substitution, throughput is

S = GP0 = Ge-2G



—This is maximum at G = 0.5, where S = 1/2e = 0.184 (frames per interval T)

•Pure ALOHA achieves low throughput


MAC Protocol: Slotted ALOHA

Slotted ALOHA is a modification to increase efficiency

—Time is divided into time slots = transmission time of a frame, T

—All stations are synchronized (eg, by periodic synchronization pulse)


MAC Protocol: Slotted ALOHA

Slotted ALOHA is a modification to increase efficiency

—Any station with data must wait until next time slot to transmit

—Any time slot with two or more frames results in a collision and loss of all frames – retransmitted after a random time


MAC Protocol: Slotted ALOHA, Cont..

“Vulnerable interval” is reduced by factor of 2 to just T

frame A

frame B

time-T 0 T

- transmission attempt at time 0

- collision if frame B was ready in interval (-T,0)



Throughput

P0 = Pr(no frames ready in previous time slot) = e-G

—Now throughput is

S = GP0 = Ge-G

—This is maximum at G = 1, where S = 1/e = 0.368 (frames per interval T)

•Slotted ALOHA doubles throughput of pure ALOHA



Note that throughput is never very high

Also, at high loads, throughput goes to 0, a general characteristic of networks with shared resources

—Number of empty time slots and successful slots decrease, number of collisions increase



—Average number of retransmissions per frame increases

—Average delay (from first transmission attempt to successful transmission) increases


MAC Protocol: CSMA

Carrier Sense Multiple Access (CSMA)

Sense the presence of carrier, sense the channel is free, send data, wait for Ack, re-send if timed-out, if busy back off and try again. Max throughput 60%

Many versions, most popular method in LAN.


MAC Protocol: CSMA, Cont..

Family of CSMA protocols defined by rules for backing off with varying degrees of persistence

—1-persistent CSMA: stations are most persistent

—P-persistent CSMA: persistence increases with value of p

—Non-persistent CSMA: stations are not that persistent


MAC Protocol: 1-persistent CSMA

Slotted or un-slotted versions

If channel is busy, station will transmit immediately after channel becomes idle

If collision is detected, then back off and try again after a random time

Propagation delay can effect performance – station A takes longer to detect that station B is transmitting

—Causes collisions to be more likely


MAC Protocol: 1-persistent CSMA, Cont..

Even without propagation delays, collisions are possible

—Stations A has the channel, stations B and C are ready and will both transmit after station A is done

Throughput analysis is complicated

—Carrier sensing improves throughput over ALOHA

—Throughput goes to 0 under very high load


MAC Protocol: P-persistent CSMA

If channel is idle, station will transmit with probability p

Otherwise, goes to next time slot and senses if channel is idle

If idle, transmits with probability p or otherwise, goes to next time slot and repeats procedure

Performance depends on choice of p


MAC Protocol: Non-persistent CSMA

If channel is idle, station will transmit

If channel is busy, station will wait for random number of time slots before trying again - even if channel is idle meanwhile

Helps avoid collisions right after an active time slot


MAC Protocol: MAC protocols, Cont..

Carrier Sense Multiple Access-Collision Detection (CSMA-CD)

—Send when carrier is free.

—Listen to detect collision

—If collision is detected, back off and retry

—Second order of improvement to CSMA


MAC Protocol: MAC protocols, Cont..

Carrier Sense Multiple Access-Collision Detection (CSMA-CD)

—Not possible in wireless LAN environment, the same frequency for sending and receiving (unlike cellular)

—CSMA-CA is the method of choice


MAC Protocol: CSMA/CD, Cont..

frame

transmission idle

frame

contention:series of time

slots for collisions

frame

time

3 alternating states: (1) transmission (2) contention (3) idle



Performance depends on time to detect collision (assume transmissions can be aborted immediately)

If D is worst-case propagation delay between any two stations, then collision detection time is 2D

station A

A begins transmit

timestation B

B begins transmit just before signal reaches B

A detects collision after 2D

signal



Assume

N = number of stations

2D = length of collision time slots

T = time to transmit frame

(T > 2D, otherwise collisions are not detected)

P = probability a station will transmit in idle time slot



After successful frame, there is contention period of series of collision time slots (multiple attempts) or idle (no attempts), ended by a successful frame (exactly one attempt)



Find P1 = Pr(exactly 1 attempt in time slot) = NP(1-P)N-1

Maximum when P = 1/N, then

Mean length of contention period:

—Pr(j slots with collisions or idle followed by one transmission) = (1 - P1)jP1

P1 1 1

N

N 1



—Mean length of contention period is

—Maximum utilization is

—Note utilization decreases for large D or small T

j(1 P1j1

) jP1 1 P1

P1

T

T 1 P1

P1

2D

(slots)

frame time

frame time + contention period=


MAC Protocol: CSMA-CA

Carrier Sense Multiple Access-Collision Avoidance (CSMA-CA)

—When node A has something to send to node B, it send Request-To-Send (RTS) packet to B with the amount of data to be sent

—B responds with Clear-To-Send (CTS) packet with time of transmission and amount of transmission back to node A


MAC Protocol: CSMA-CA

Carrier Sense Multiple Access-Collision Avoidance (CSMA-CA)

—When a node has something to send, it should also checks CTS before start transmitting.

—Improves CSMA performance


What Is Hidden Node?

A CB

A can hear BC can hear BA can not hear CC can not hear A sending data


MAC Frame Format

CRCFrameControl

Duration SequenceControl

FrameBody

Address 4Address 1 Address 2

ProtocolVersion

Type Sub type To DS

FromDS

RetryLastFragment

RSVDEPPower Mgt

2 6 6 6 2 2 0-2304 4

2 2 4 1 1 1 1 2 1 1


Frame type and subtypes

Three type of frames—Management

—Control

—Asynchronous data

Each type has subtypes

Control—RTS

—CTS

—ACK


Frame type and subtypes, Cont..

Management—Association request/ response—Re-association request/ response—Probe request/ response—privacy request/ response—Beacon (Time stamp, beacon interval, TDIM period,

TDIM count, channels sync info, ESS ID, TIM broadcast indicator)

—TIM (Traffic Indication Map) indicates traffic to a dozing node

—dissociation—Authentication


Power Management

AP knows the power management of each node

AP buffers packets to the sleeping nodes

AP send Traffic Delivery Information Message (TDIM) that contains the list of nodes that will receive data in that frame, how much data and when.

The node is awake when it is sending data, receiving data or listening to TDIM.


Authentication

Three levels of authentication

—Open: AP does not challenge the identity of the node.

—Password: upon association, the AP demands a password from the node.

—Public Key: Each node has a public key. Upon association, the AP sends an encrypted message using the nodes public key. The node needs to respond correctly using it private key.


Mobility Management

Access point connects other access points via backbone network

Backbone Network

Access Point

Access PointAccess Point


Mobility Management, Cont..

A node can associate when it enters the coverage area of an AP

It shall re-associate when it handoffs to another AP.

AP bridge function keeps track of all nodes associated with it.


Access Point Functions

Access point has three components

—Wireless LAN interface to communicate with nodes in its service area

—Wireline interface card to connect to the backbone network

—MAC layer bridge to filter traffic between sub-networks. This function is essential to use the radio links efficiently


Bridge Functions

Listen to all packets being sent.

Find out which nodes are in which sub-network by analyzing the source address. Store that data in a routing table.

If a packet is addressed to a known node, only repeat the data on that sub-network, otherwise repeat it on all networks.

Age the entries after a timer value has expired since the last communication


Bridge Functions, Cont..

If the timer is too long, we might send data to a node that might have left the sub-network or is turned off or even gone to coverage area of another access point.

If the timer is too short, we remove the user too early and repeat the packet destined to it in all sub-networks.

Other functions of a bridge, buffering for speed conversion, changing frame format between LANs.


Routing

Building routing tables can be done as

—Source tree, keeps track where other nodes are and the best way of reaching them. When sending a packet the route is also determined. It must be done in each node and is heavy.

—Spanning tree, is built iteratively, each bridge advertises it identity and all other bridges it knows and how many hops it takes to get there. Then each bridge follows a specific algorithm to calculate how get to each bridge with least hop.


IEEE 802.11 WLAN

1997 IEEE 802.11 working group developed standard for inter-working wireless LAN products for 1 and 2 Mbps data rates in 2.4 GHz ISM (industrial, scientific, and medical) band (2400-2483 MHz)

Required that mobile station should communicate with any wired or mobile station transparently (802.11 should appear like any other 802 LAN above MAC layer), so 802.11 MAC layer attempts to hide nature of wireless layer (eg, responsible for data retransmission)


802.11 WLAN, Cont..

1999 IEEE 802.11a amendment for 5 GHz band operation and 802.11b amendment to support up to 11 Mbps data rate at 24 GHz

MAC sublayer uses CSMA/CA (carrier sense multiple access with collision avoidance) - very similar to CSMA/CD except collisions are detected by ACKs after entire packets are transmitted


802.11 WLAN, Cont..

—Station will listen to channel if ready to transmit

—If channel is idle, begins to transmit

—If channel is busy, will wait until channel is free and transmit after a random time (to reduce collisions)

—In case of collisions, stations will try again following a random exponential back off


802.11 WLAN, Cont..

Random exponential back off:

—Stations keep track of contention window parameter, CW

• Initially CW is a minimum value

—When station wants to transmit, it chooses a random (uniformly likely) value between 0 and CW


802.11 WLAN, Cont..

—Waits for chosen number of time slots before transmitting

—After each collision, CW is doubled (exponential increase)

X

collisions

tries in oneof 2 slots

Example X X




802.11 WLAN, Cont..

Other MAC sublayer functions:

—Optional “point coordination function”: centralized contention-free multiple access for time-sensitive data (a centralized polling mechanism)

—“Association” and “re-association” processes to dynamically establish connections between mobile stations and fixed access points


802.11 WLAN, Cont..

—Optional encryption for security

—Power management to allow mobile stations to power down (sleep) without losing data (eg, access point will buffer packets for sleeping stations until requested)


HIPERLAN

WLANs have small share of LAN market now—Higher costs per station—Standards are recent (HIPERLAN, IEEE

802.11)—Rapid growth is projected

1995 ETSI technical group RES 10 (Radio Equipment and Systems) developed HIPERLAN/1 wireless LAN standards using 5 channels in 5.15-5.3 GHz frequency range—Technical group BRAN (Broadband Radio Access

Network) is standardizing HIPERLAN/2 for wireless ATM


HIPERLAN, Cont..

HIPERLANs with same radio frequencies might overlap

—Stations have unique node identifiers (NID)

—Stations belonging to same HIPERLAN share a common HIPERLAN identifier (HID)

—Stations of different HIPERLANs using same frequencies cause interference and reduce data transmission capacity of each HIPERLAN

—Packets with different HIDs are rejected to avoid confusion of data


HIPERLAN, Cont..

Data link layer = logical link control (LLC) sublayer + MAC sublayer + channel access control (CAC) sublayer

data link

physical

LLC

MAC

network

CAC


HIPERLAN, Cont..

MAC sublayer:—Keeps track of HIPERLAN addresses (HID

+ NID) in overlapping HIPERLANs—Provides lookup service between network

names and HIDs—Converts IEEE-style MAC addresses to

HIPERLAN addresses—Provides encryption of data for security


HIPERLAN, Cont..

MAC sublayer:—Provides “multi hop routing” – certain

stations can perform store-and-forwarding of frames

—Recognizes user priority indication (for time-sensitive frames)


HIPERLAN, Cont..

CAC sublayer:—Non-preemptive priority multiple access

(NPMA) gives high priority traffic preference over low priority

—Stations gain access to channel through channel access cycles consisting of 4 phases:


HIPERLAN, Cont..

CAC sublayer:1. Priority phase: if station has data with

priority N, will wait for N-1 priority slots and transmit “priority assertion” burst in Nth slot

• If it hears another station of higher priority, it will give up on this channel access cycle

•Winning stations of same priority go into next phase


HIPERLAN, Cont..

2. Elimination phase: each station will transmit a burst of random length (geometrically distributed number of time slots) and see if channel is idle

• If channel is idle, it will go to next phase

• If busy, it will give up on this access cycle


HIPERLAN, Cont..

3. Yield phase: surviving stations will listen to channel for a random number of time slots (geometrically distributed)

• If it hears another station transmitting, it will give up on this access cycle

• If channel is idle, it will begin to transmit


HIPERLAN, Cont..

4. Transmission phase: winning station will transmit

• CAC is designed to give each station (of same priority) equal chance to access the channel

#1EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU Southern Methodist University Fall 2003 EETS 8316/NTU CC745-N Wireless Networks Lecture 8: Mobile Data,

Documents

ntu tc

gprs interfaces

engineeringsmu review

gprs register

gprs capable

gprs subscriber data

sgsn slide

mobile data